The market of display units has expanded rapidly with the rising popularity of personal computers and the like, making display units become commonplace items. In such a situation, it has recently been required to suppress a leakage magnetic field generated around display units to a specified value or less. In connection with such a requirement, there is MPR-II Standard as a standard for a VDU measurement, which is an international standard. MPR-II Standard prescribes that a leakage magnetic field in a VLF band of 2 kHz to 400 kHz should be suppressed to 25 nT or less.
One of the main sources of leakage magnetic fields in an ELF band with a frequency range of 5 Hz to 2 kHz and in a VLF band with a frequency range of 2 kHz to 400 kHz around a display unit is a deflection yoke mounted in a display unit. Various measures have been employed for reducing the amount of a generated leakage magnetic field. Particularly, special measures are required to reduce the amount of a leakage magnetic field in a VLF band. One method for reducing a leakage magnetic field involves attaching one cancel coil or a plurality of cancel coils to the deflection yoke. This kind of deflection yoke is disclosed, for example, in Unexamined Japanese Patent Application Tokkai Hei 5-190115.
FIG. 11 is a cross-sectional view showing a deflection yoke provided with a means for reducing a leakage magnetic field in the prior art. This deflection yoke comprises a separator 101, a horizontal deflection coil 102 that is wound on the separator 101, a ferrite core 103 that is arranged so as to cover the center portion of the separator 101, a cancel coil bobbin 104 that is arranged so as to cover the front-edge side of the horizontal deflection coil 102, and a cancel coil 105 that is wound with the cancel coil bobbin 104. This deflection yoke is mounted at a predetermined position outside a cathode-ray tube in a display unit. In this case, the cancel coil 105 is formed in a loop shape, and a pair of the cancel coils are provided at the upper and lower portions on the front-edge side of the deflection yoke body. A cancel magnetic field generated on the tube axis of the cathode-ray tube by the cancel coils 105 has two peaks with a reverse polarity to those of a deflection magnetic field generated at the front portion and the rear portion of the deflection yoke body. The cancel magnetic field has a distribution characteristic in which the ratio of the values of the two peaks is substantially the same.
However, in the display unit according to the prior art described above, a pair of cancel coils must be provided at the upper and the lower portions on the front-edge side of the deflection yoke body in order to suppress a leakage magnetic field. Therefore, the number of parts and of manufacturing processes increase. This generally results in low working efficiency in manufacturing the deflection yoke, which eventually leads to a low working efficiency in manufacturing the display unit and a corresponding increase in the cost of the display unit. In the prior art described above, there have been some cases where the leakage magnetic field in a VLF band of 2 kHz to 400 kHz, which is regulated by MPR-II Standard, cannot be suppressed to the specified value or less at low cost.
An actual deflection coil has a cone-shape with a larger diameter at the front side (at a face panel side of a cathode-ray tube). For the purpose of improving the quality of display images in a cathode-ray tube, strong distortion is partially provided to a deflection magnetic field. Therefore, a leakage magnetic field does not generally have a symmetry property relative to the center of a deflection yoke due to the influence of an auxiliary coil provided at the rear-end part of the deflection yoke in many cases. In such cases, the leakage magnetic field cannot be cancelled effectively by a cancel magnetic field having two peaks that are symmetric at the front portion and the rear portion of the deflection yoke body. Specifically, when trying to cancel the leakage magnetic field at the front side of a face panel optimally, the cancel magnetic field strengthens the leakage magnetic field at the rear side of a cathode-ray tube in some cases.